Induction motors are used to provide mechanical power in a variety of applications such as, for example, driving traction devices, wind tunnels, pipeline compressors, washing machines, dishwashers, standalone fans, and record players. Induction motors are electrically driven by a current to generate a mechanical torque output. That is, changing currents within components (e.g. field windings) of the induction motor create an electromagnetic flux, and a rotating member of the induction motor is caused to rotate by one or more currents interacting with the magnetic flux (e.g. in accordance with the principle of Lorentz force). The current applied to an induction motor can be regarded as a vector with two components: a torque current component iq, and a flux current component id. By controlling these current components using associated respective voltages applied to the induction motor, the electromagnetic flux and output torque of the induction motor can be controlled. For example, voltage commands can be delivered to an inverter to control the inverter to produce one or more desired voltages. The desired voltages can then be applied to the induction motor to cause desired torque and flux current components to flow within the induction motor, thus inducing a desired mechanical torque output from the induction motor in accordance with known principles (e.g. Lorentz force).
During normal operation of an induction motor, the induction motor will generate a counter-electromotive force (“back EMF”) which opposes the voltage applied to the induction motor. There is a speed threshold above which the back EMF becomes greater than the voltage applied to the induction motor, which can cause the induction motor to operate undesirably or to cease operation entirely. A method known as “field weakening” is employed to reduce the back EMF (i.e. maintain the back EMF at or below a desired level for operation of the induction motor) to allow the induction motor to operate at high speeds (e.g. speeds faster than the speed threshold).
Field weakening methods generally include reducing the flux of an induction motor when the induction motor speed increases beyond a predetermined speed threshold, and maintaining the flux of the induction motor at or above a normal flux level when the induction motor speed is below a predetermined threshold (i.e. when the induction motor is experiencing a normal operation) to increase the operational efficiency of the induction motor. For example, this can be accomplished by delivering voltage commands to an inverter such that the flux current component delivered to the induction motor is reduced. Field weakening methods can further include using the voltage commands to increase the torque current component such that the mechanical torque output of the induction motor can be maintained at or above a level for efficient operation of the induction motor. However, during aggressive acceleration/deceleration events (i.e. relatively fast changes in motor speed), the traditional field weakening method reacts relatively slowly to the fast changes in motor speed and causes the motor to operate undesirably. For example, the motor may generate an undesirable output torque and thus function inefficiently, and/or experience overmodulation (i.e. distortions in electrical waveforms associated with operation of the motor). Thus, there is a need for a field weakening control method that improves the transient performance capability of an induction motor during aggressive acceleration/deceleration events while maintaining the efficiency of the induction motor during other field weakening events (i.e. high speed events) and normal motor operation.
One field weakening control method for handling an induction motor during an excessive state of speed increase or decrease is disclosed in U.S. Pat. No. 6,104,159 (“the '159 patent”) issued to Seok. Specifically, the '159 patent discloses a field weakening control method wherein a voltage limit is established in an oval on a current plane and a current limit is established in a circle on the same current plane. The current plane has a flux current portion id along its x-axis and a torque current portion iq along its y-axis such that each point on the current plane corresponds to a respective current command. The circle is centered about the origin of the current plane and the oval is a translation δ units in the negative id direction of an oval centered about the origin. A major axis of the oval is parallel with the y-axis of the current plane. The intersection of the oval and the circle corresponds to those current commands used during excessive states of the induction motor, while a region not within the intersection of the oval and the circle corresponds to those current commands used during other states of the induction motor. Within the intersection of the oval and the circle, desired current values ieds and ieqs for the flux current portion and the torque current portion, respectively, are calculated according to the following formulas:
            i      ds      e        =                            -                      L            s                          ⁢        δ        ⁢                                                            (                                                      L                    s                                    ⁢                  δ                                )                            2                        +                                          (                                                      L                    s                    2                                    -                                      L                    σ                    2                                                  )                            ⁢                                                (                                                            V                                              s                        ⁢                                                                                                  ⁢                        max                                                              /                                          ω                      e                                                        )                                2                                      -                          δ              2                        -                                          L                σ                2                            ⁢                              I                                  s                  ⁢                                                                          ⁢                  max                                2                                                                          L          s          2                -                  L          σ          2                          where:        δ    =                            L          m                          L          r                    ⁢      α      ⁢                          ⁢              ⅇ                  1          τ1                                i      qs      e        =                            I                      s            ⁢                                                  ⁢            max                    2                -                  i                      ds            ⁢                                                                      e            ⁢                                                  ⁢            2                              where Ls is a stator self inductance, Lσ is a leakage inductance, Lm is a magnetizing inductance, Lr is a rotor self inductance, and ωe is an excitation angular frequency. Within the region not included in the intersection of the oval and circle, desired current values for the flux current portion and the torque current portion are calculated according to the following formulas:
            i      ds      e        =                            -          b                +                                            b              2                        -                          4              ⁢              a              ⁢                                                          ⁢              c                                                  2        ⁢        a                  where:              a      =              2        ⁢                  L          m                ⁢                  L          s          2                      ,                  ⁢          b      =                                    L            s            2                    ⁢                      αⅇ                                          -                1                            τ                                      +                  3          ⁢                      L            m                    ⁢                      L            s                    ⁢          δ                      ,                  ⁢          c      =                                    L            s                    ⁢                      δαⅇ                                          -                1                            τ                                      +                              L            m                    ⁢                      δ            2                          -                                            L              m                        ⁡                          (                                                V                                      s                    ⁢                                                                                  ⁢                    max                                                  /                                  ω                  e                                            )                                2                                i      qs      e        =                                                      (                                                V                                      s                    ⁢                                                                                  ⁢                    max                                                  /                                  ω                  e                                            )                        2                    -                                    (                                                                    L                    s                                    ⁢                                      L                    ds                    e                                                  +                δ                            )                        2                                      L        σ            where Ls is a stator self inductance, Lm is a magnetizing inductance, and ωe is an excitation angular frequency.
While the field weakening control method of the '159 patent may increase efficiency of an induction motor during excessive acceleration/deceleration states, it may be expensive to use. More specifically, because the field weakening control method requires the execution of several complicated mathematical formulas in order to determine desired current commands, it may require one or more relatively expensive processors and/or microprocessors equipped with high-end arithmetic units and/or other components for processing complicated mathematical formulas.
Further, the complicated formulas may require a large amount of processor resources, thus preventing the processor or microprocessor using the method from executing other tasks efficiently. For example, an engine control module may fail to regulate fuel injection for an undesirably long amount of time while processing the formulas of the '159 patent. Alternatively, a dedicated processor or microprocessor could be used to implement the method of the '159 patent, but the dedicated processor or microprocessor may be expensive, as discussed above.
Further still, the field weakening control method of the '159 patent may be difficult to implement. That is, because the method of the '159 patent requires that the stator self inductance, leakage inductance, magnetizing inductance, and rotor self inductance be known in order to calculate the desired current commands, it may be difficult or impossible to use a single unit implementing the method with two induction motors having different values for those constants.
The present disclosure is directed to overcoming one or more of the problems set forth above.